For the past few years, about 1,500 people in the Philadelphia area have worn small electronic devices that keep track of every bit of radio programming they might hear in the car, at the office, as they walk to lunch, when they drop by a friend's house or when they hit happy hour after work.

The gadget, the Portable People Meter, contains motion detectors to make certain that someone is really wearing the device, and extra-sensitive microphones that pick up codes hidden in each program. Each evening, the wearer takes off the device, plugs it into a docking station and transmits the data to Arbitron, the company that has dominated the radio ratings business for decades.

The Portable People Meter is the next big thing in ratings. While Media Matrix and Nielsen use software that monitors every click on the computers of people who have agreed to be part of the Internet ratings sample, and Nielsen attaches electronic meters to the TVs in the 5,000 homes that make up the national ratings sample, Arbitron's radio ratings still rely entirely on paper diaries that listeners fill out by hand.

It is hard to find anyone in radio who believes the Arbitron ratings really reflect what people listen to. The diaries are too subject to embarrassment (Do I really want to write down that I listen to all five hours of Howard Stern?), boasting (Oh yeah, I listen to the news, all day), and memory lapses (Which one of those hit music stations was on in the car?).

But the biggest problem is that radio ratings -- and therefore nearly all radio programming decisions -- are based on diaries that an ever-dwindling portion of the public is willing to fill out. The number of people who agreed to take $2 a week to complete a diary dropped from 43 percent of those asked in 1995 to 34 percent in 2002. Arbitron has upped the payoff to $10 a week, but the basic problem remains: Diaries ask listeners to reconstruct their day (or week), remembering fine details about how many minutes they spent with each radio station.

The people meter is still a few years from nationwide deployment, but eventually it will track not only radio listening, but every kind of media exposure, from TiVo to Web surfing, cable to satellite. And fears of Big Brother are no hindrance: Finding enough people to wear the meter is no problem, the ratings companies say. The token payment hardly matters; it's the thrill of being counted (with minimal effort expended) that draws people in.

Ron Kolessar, Arbitron vice president, holds the new "Portable People Meter," at the Arbitron lab in Columbia.The meters and their chargers are a few years away from national use.